High nonlinear optical anisotropy of urea nanofibers D. Isakov, E. de Matos Gomes, M. Belsley, B. Almeida, A. Martins, N. Neves and R. Reis EPL, 91 2 (2010) 28007 Published online: 05 August 2010 DOI: 10.1209/0295-5075/91/28007